1. Field of the Invention
The present invention relates to a laser diode using group III nitride group compound semiconductor. Here a group III nitride group compound semiconductor is represented by a general formula AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61), which includes binary compounds such as AlN, GaN and InN, ternary compounds such as AlxGa1xe2x88x92xN, AlxIn1xe2x88x92xN and GaxIn1xe2x88x92xN (0 less than x less than 1), and quaternary compounds such as AlxGayIn1xe2x88x92xxe2x88x92yN (0 less than x less than 1, 0 less than y less than 1, 0 less than x+y less than 1). In this specification, a group III nitride group compound semiconductor includes a group III nitride group compound semiconductor which is doped with impurities to have p-type or n-type conductivity.
2. Description of the Related Art
A group III nitride group compound semiconductor is a direct-transition-type semiconductor having a wide emission spectrum range from ultraviolet to red, and is applied to light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs). The group III nitride group compound semiconductor is, in general, formed on a sapphire substrate. A laser diode, in general, comprises a guide layer and a cladding layer, which are formed on an n-type and a p-type semiconductor side of an active layer, respectively, sandwiching the same. The cladding layer is formed to have a large band gap and is generally made of AlxGa1xe2x88x92xN (0 less than x less than 1) including aluminum (Al), in order that electrons and holes injected from the negative and the positive electrode, respectively, form electron-hole pairs in the active layer. The guide layer preferably has a wider band gap than the active layer. The guide layer is preferably made of, e.g., gallium nitride (GaN) in order that laser light can be confined in the active layer by total internal reflection due to the difference in refractive indices. The active layer preferably has a multiple quantum well (MQW) structure.
FIG. 5 illustrates the structure of a laser diode (LD) 900 as a conventional group III nitride group compound semiconductor light-emitting device. The laser diode (LD) 900 comprises a substrate 901, and an AlN buffer layer 902 is formed thereon.
On the buffer layer 902, the following four layers are formed successively: an n-layer 903 made of silicon (Si) doped GaN; an n-cladding layer 904 made of silicon (Si) doped AlxGa1xe2x88x92xN; an n-guide layer 905 made of silicon (Si) doped GaN; and an active layer 906 having a multiple quantum well (MQW) structure in which a barrier layer made of GaN and a well layer made of Ga1xe2x88x92yInyN are laminated alternately. On the active layer 906, a p-guide layer 907 made of magnesium (Mg) doped GaN, a p-cladding layer 908 made of magnesium (Mg) doped AlxGa1xe2x88x92xN, and a p-contact layer 909 made of magnesium (Mg) doped GaN are formed. An electrode 910A is formed on the p-contact layer 909 and another electrode 910B is formed on a portion of the n-layer 903.
In the above-described conventional technique, however, stress between the sapphire substrate 901 and the n-layer 903 or the n-cladding layer 904 is applied to the active layer 906 through the n-guide layer 905. As a result, luminous efficiency of the active layer 906 decreases and oscillation threshold current of the laser increases.
An object of the present invention is, therefore, to provide a laser diode using group III nitride group compound semiconductor, which has a guide layer with decreased elastic constant.
To achieve the above object, and others, a first aspect of the present invention provides a laser diode using a group III nitride group compound semiconductor diode. The diode comprises a guide layer having a multiple layer structure including an indium nitride (InN) layer and is formed on a substrate side of an active layer.
A second aspect of the present invention provides a diode having a guide layer which has a multiple layer structure including an indium nitride (InN) layer which is disposed on the side of the active layer opposite to the substrate.
A laser diode using a group III nitride group compound semiconductor comprises a substrate and group III nitride group compound semiconductor layers laminated thereon. By forming a guide layer which has a multiple layer structure including an indium nitride (InN) layer and has a sufficiently lowered elastic constant beneath an active layer, the elastic constant of the guide layer becomes comparatively smaller. As a result, transmission of stress, which is generated by temperature variation during manufacture or use, can be prevented. When a guide layer which comprises a multiple layers including indium nitride (InN) is also formed on an active layer opposite to the substrate side, further improvement can be obtained. Because of the structure of the emission layer, the laser diode of the present invention can be used to produce ultraviolet light. The guide layers are preferably the multiple layers including InN and a group III nitride group compound semiconductor including no indium (In), for example, AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61). For example, the guide layers preferably comprise the multiple layers including InN and AlxGa1xe2x88x92xN (0 less than x less than 1) or, InN and GaN.